But answers to such questions depend on eventually detecting these predicted meganeutrinos.

Although they should be extraordinarily common in the universe, the relic neutrinos now have only about one ten-billionth of the energy of neutrinos generated by the sun.

"This makes relic neutrinos near impossible to detect directly, at least with anything one could build on Earth," study co-author Fuller said.

Still, the fact that there are so many relic neutrinos means that together they likely exert a significant gravitational pull—"enough to be important for how the universe as a whole behaves," Fuller added.

Dark matter, for example, has never been directly observed. But astrophysicists have found proof that dark matter exists based on its effect on colliding galaxies.

"So by looking at the growth of structures in the universe," Fuller said, "you might be able to detect relic neutrinos indirectly by their gravity."